mute_time_intervals should be inherited to sub routes

What did you do?
I set up a route to mute all warning alert outside work days, and several sub routes to route these alerts to different receivers.

What did you expect to see?
I was expecting this alert could be silenced in all sub routes

What did you see instead? Under which circumstances?
the alert was sent regardless of the mute intervel setting
Environment

• System information:

  insert output of uname -srm here

• Alertmanager version:

  insert output of alertmanager --version here (repeat for each alertmanager
  version in your cluster, if relevant to the issue)
  v0.23.0

• Prometheus version:

  insert output of prometheus --version here (repeat for each prometheus
  version in your cluster, if relevant to the issue)

• Alertmanager configuration file:

insert configuration here

• Prometheus configuration file:

insert configuration here (if relevant to the issue)

• Logs:

insert Prometheus and Alertmanager logs relevant to the issue here

I'm aware the document said:

# Times when the route should be muted. These must match the name of a
# mute time interval defined in the mute_time_intervals section. 
# Additionally, the root node cannot have any mute times.
# When a route is muted it will not send any notifications, but
# otherwise acts normally (including ending the route-matching process
# if the `continue` option is not set.)
mute_time_intervals:
  [ - [<string>](https://www.prometheus.io/docs/alerting/latest/configuration/#string) ...]

but I think these rules should be inherited, this can make the config shorter and easier to read, otherwise I have to set mute time for every single route.

Start with Alertmanager's routing configuration entry point and the mute_time_intervals documentation referenced in the issue, then trace how parent and sub-route settings are evaluated. Done means a mute interval configured on a parent route applies to its sub routes as expected, with coverage for the inheritance behavior.